A known electric power steering apparatus for turning steerable wheels of a vehicle includes an input member to which steering torque is applied, a steering torque sensor operatively coupled with the input member, an electronic control unit, and an electric motor. The electronic control unit receives signals from the torque sensor, and possibly other sensors, and controls the electric motor.
The electric motor has a motor shaft which rotates about a motor axis. The motor shaft is connected with a gearbox which provides a gear reduction between the motor shaft of the electric motor and an output pinion meshed with a linearly movable rack. Rotation of the output pinion causes the rack to move linearly to turn the steerable wheels.
The gearbox typically includes a pair of meshed gears, such as a worm gear set. Some amount of backlash may be present between the pair of gears, whatever the configuration of the gears, in the gearbox. The backlash, or amount by which the width of a tooth space exceeds the thickness of an engaging tooth measured on the pitch circle, results primarily from manufacturing tolerances. Excessive backlash can generate undesirable noise and cause the gears to wear. It is desirable for an electric power steering apparatus to include a means for adjusting backlash of the gears in the gearbox between the electric motor and the output pinion.